<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>固定单位布局</title>
    <style>
        #demo1 {
            width: 400px;
            height: 400px;
            background-color: skyblue;
        }

        #demo2 {
            width: 200px;
            height: 200px;
            background-color: gray;
        }
    </style>
</head>

<body>
    <div id="demo1">
        <div id="demo2"></div>
    </div>
</body>

</html>